The vasomotor component of the orienting response in man related to spontaneous fluctuation of vasomotor activity in the acral skin zone.

A plethymographic study of the effect of a weak indifferent light or sound stimulus and of a physically identical light stimulus with signal meaning on vasomotor activity in the acral skin zone of man has shown that during stimulus presentation there was a significant rise in the percent of occurrence of vasmotor waves corresponding to the criteria for the vasomotor orienting response (VOR). VOR is elicited significantly more often by stimuli with signal meaning than by physically identical indifferent stimuli. After the acute extinction or VOR disinhibition followed in all cases, and after 40--60 repeated stimuli the occurrence of VOR was still significant. Repeated stimuli had a significant effect on the course of the plethysmographic curve, especially on the component of waves of third order, changing the frequency spectrum and time contingency of waves.